1. Field of the Invention
The present invention relates to a method for measuring the azimuthal strike orientation of induced fractures in subterranean formations from which the maximum and minimum in situ stress direction can be inferred. More particularly, the present invention relates to a method for direct measurement of the azimuthal strike orientation of induced fractures by using an oriented core and computed tomography imagery. The method of the present invention can also be extended to the direct measurement of the spatial orientation of other planar rock fabrics causing mechanical rock anisotropy which can be compared to the induced fracture orientation.
2. Background
The ability to predict and/or measure hydraulic fracture orientation and in situ stress direction in an oil and gas reservoir is important for optimum field development in hydraulically stimulated reservoirs, for well placement, stimulation design, injection of fluids and is important for decisions for optimum placement of horizontal oil and gas wells. Knowing the fracture direction allows the field well spacing to be determined, and the shape of the drainage area to be established. Several methods exist in the oil and gas industry for measuring, or at least inferring, hydraulic fracture orientation in subterranean formations and for inferring the direction of the maximum horizontal in situ stress.
One of the previously known methods for determining fracture orientation involved performing an open hole microfrac test in a well, and thereafter, taking an oriented core sample from the bottom of the well bore and visually observing the direction of the fractures induced during the microfrac test.
Another prior art technique for inferring fracture orientation involves the use of anelastic strain ("ASR") techniques. An ASR test consists of immediately sectioning and placing in a test apparatus a portion of a freshly cut and recovered oriented core section for recording the expansion/contraction of the rock due to the release of the stress pattern it has been under in place. The core is placed in a test fixture and the minute oriented displacements recorded for 24 to 48 hours, until movement ceases. Since the stress within a formation is proportional to the strain relaxation in the core sample, the direction of the minimum and maximum horizontal stress within the formation may be inferred from the relaxation data.
A recently developed non-prior art technique for determining fracture orientation is through use of an acoustic scanning tool (CAST) to determine fracture orientation after fractures have been induced in the formation by an open hold microfrac test. The CAST is an oriented sonic tool that may be used to observe the interior of a well bore. Observation of the induced fractures with the CAST allows an operator to directly observe the orientation of both natural and induced fractures.
Yet another new non-prior art method for measuring the direction of hydraulic fracture orientation has been developed. This technique involves the use of downhole extensiometers to measure borehole diameter changes before and after fractures have been initiated in the formation. This method and technique is the subject of a separately filed application which is assigned to the assignee of the present invention (application Ser. No. 07/902,108, filed Jun. 22, 1992). A downhole extensiometer such as Halliburton Services, Inc.'s THE.TM. tool is an instrument which measures borehole deformation during a fracture. THE.TM. tool is a high precision oriented, multi-armed caliper with a high accuracy memory pressure gauge. This tool will be in the hole during a microfracture treatment to measure the actual fracture width created. THE.TM. tool uses straddle packers to isolate and test individual zones in an open hole. Deformation of the borehole will give an indication of the azimuthal strike orientation of the induced fracture.
The data from the existing prior art methods is often not available, lacks verification, is only obtained as a single measurement lacking statistical certainty or is inferred from indirect techniques which can be difficult to interpret. The method of the present invention is one which may provide verification of these other methods in a particular field and is a direct measurement which can be coupled with several of the existing methods. The method of the present invention can also be extended to the direct measurement of the planar rock fabrics causing mechanical rock anisotropy which can be compared to the hydraulic fracture orientation.
The method of the present invention provides a direct measurement of azimuthal strike orientation of induced fractures from which maximum and minimum stress directions can be inferred. Maximum in situ stress is shown to be aligned parallel to the strike orientation of induced core fractures. A hydraulically induced fracture will propagate perpendicular to the least principal stress and in the direction of the greatest principal stress. The proposed method requires the use of an oriented whole core and computed tomography (CT) imagery.
Coring and core orientation techniques are well-known in the industry. One such technique for core orientation includes the use of a downhole camera and compass. Orientation data is obtained by taking photographs of the downhole compass at desired intervals over the cored section. By way of example, downhole compass photographs are obtained every three feet through the section being cored. Rotation of the core bit is stopped at the desired depth to obtain a readable photograph of the downhole compass.
Orientation grooves, the principal and secondary scribe lines, are marked on the core as the core is being cut. Knives inside the core barrel cut the scribe lines as the core enters the core barrel. The orientation of the principal scribe with respect to the compass is recorded prior to running the core barrel into the borehole. Thus, one can determine the orientation of the principal scribe line from the compass readings at each recorded interval. The secondary scribe lines are used as a reference for identifying the principal scribe. A survey record will exist at the conclusion of the cored section which accurately reflects the orientation of the core's principal scribe line throughout the interval. Orientation of the core is considered a critical part of obtaining accurate orientation measurements of planar core features such as fractures. State of the art continuous orientation technology which is now available to the industry is an alternative to "camera" technique of core orientation described above.
Computed tomography (CT), commonly known in the medical field as CAT scanning ("computerized axial tomography" or "computer assisted tomography"), is a nondestructive technology that provides an image of the internal structure and composition of an object. What makes the technology unique is the ability to obtain imaging which represents cross sectional "axial" or "longitudinal" slices through the object. This is accomplished through the reconstruction of a matrix of x-ray attenuation coefficients by a dedicated computer system which controls the scanner. Essentially, the CT scanner is a device which detects density differences in a volume of material of varying thicknesses. The resulting images and quantitative data which are produced reflect volume by volume (voxel) variations displayed as gray levels of contrasting CT numbers.
Although the principles of CT were discovered in the first half of this century, the technology has only recently been made available for practical applications in the non-medical areas. Computed tomography was first introduced as a diagnostic x-ray technology for medical applications in 1971, and has been applied in the last decade to materials analysis, known as non-destructive evaluation. The breakthroughs in tomographic imaging originated with the invention of the x-ray computed tomographic scanner in the early 1970's. The technology has recently been adapted for use in the petroleum industry.
A basic CT system consists of an x-ray tube; single or multiple detectors; dedicated system computer system which controls scanner functions and image reconstructions and post processing hardware and software. Additional ancillary equipment used in core analysis include a precision repositioning table; hard copy image output and recording devices; and x-ray "transparent" core holder or encasement material.
A core is laid horizontally on the precision repositioning table. The table allows the core to be incrementally advanced a desired distance thereby ensuring consistent and thorough examination of each core interval. The x-ray beam is collimated through a narrow aperture (2 mm to 10 mm), passes through the material as the beam/object is rotated and the attenuated x-rays are picked up by the detectors for reconstruction. Typical single energy scan parameters are 75 mA current at an x-ray tube potential of 120 kV. After image reconstruction, a cross-sectional image is displayed and the data stored on tape or directly to a computer disk. One example of obtaining image output is through hard copies in the form of 35 mm slides directly from image disks which may then be reproduced into 8.5.times.11 inch photographic sheets directly from the slides.
A cross sectional slide of a volume of material can be divided into an n x n matrix of voxels (volume elements). The attenuated flux of N.sub.o x-ray photons passing through any single voxel having a linear attenuation coefficient .mu. reduces the number of transmitted photons to N as expressed by Beer's law: EQU N/N.sub.o =e.sup.-.mu./x
where:
N=number of photons transmitted PA1 N.sub.o =original number of emitted photons PA1 x=dimension of the voxel in the direction of transmitted beam PA1 .mu.=linear attenuation coefficient (cm). PA1 (.mu./.rho.).sub.w =mass attenuation coefficient of water PA1 .rho..sub.w =density of water
Material parameters which determine the linear attenuation coefficient of a voxel relate to mass attenuation coefficient as follows: EQU .mu.=(.mu./.rho.).rho.
where: (.mu./.rho.) is the mass attenuation coefficient (MAC) and .rho. is the object density.
Mass attenuation coefficients are dependent on the mean atomic number of the material in a voxel and the photon energy of the beam [approx. (KeV).sup.-3 ]. For a heterogeneous voxel, i.e., compounds and mixtures, the atomic number depends on the weighted average of the volume fraction of each element (partial volume effect). Therefore, the composition and density of the material in a voxel will determine its linear attenuation coefficient.
Computed tomography calculates the x-ray absorption coefficient for each pixel as a CT number (CTN), whereby: ##EQU1## where: .mu..sub.w is the linear attenuation coefficient of water.
Conventionally, CT numbers are expressed as normalized MAC's to that of water. The units are known as Hounsfield units (HU) and are defined as O HU for water and (-1000) HU for air. Rearrangement of the previous equation can therefore be expressed as: EQU CTN (CT number)=1000.times.((.mu./.rho.).rho./(.mu./.rho.).sub.w .rho..sub.w -1)
where:
Core lithology can be determined by single scan CT with the knowledge of the density (or grain density) and attenuation coefficient of the material. For sandstones, limestones, and dolomites, the grain densities are usually close to the literature values (2.65, 2.71, and 2.85 g/cm.sup.3, respectively). Typical densities can also be used for rock of mineral types such as gypsum, anhydrite, siderite, and pyrite.
The mass attenuation coefficients of various elements and compounds can be found in the nuclear data literature. The mass attenuation coefficient for composite materials can be determined from the elemental attenuation coefficients by using a mass weighted averaging of each element in the compound as shown: ##EQU2## where M.sub.i is the molecular weight for element i.
Note that calcite MAC values are higher than those for dolomite, even though dolomite has a higher grain density than calcite. This is because of the atomic number dependence. Water and decane have very similar MAC values. The higher atomic number (and MAC value) materials are more nonlinear with x-ray energy than the lower atomic number materials.
In general, sandstones or silicon-based materials have CT numbers in the 1000-2000 range, depending on the core porosity. Limestones and dolomites are typically in the 2000-3000 CTN range.
Small impurities of different elements in a core can change the core's CT numbers. For instance, the presence of calcium in a sandstone core maxtrix will increase the core's CT number above what would be predicted from the porosity vs. CTN curve. An estimate of the weight fraction of each element in the core can give a better estimate of the core porosity.
The occurrence of abrupt changes in CT number may indicate lithology discontinuities in the core. For instance, the presence of small high density/high CT number nodules (CTN&lt;2000) usually indicates the presence of iron in the core (pyrite, siderite, glauconite). For limestones the presence of higher density/CTN nodules (CTN&gt;3400) in the limestone matrix may indicate anhydrite in the core. A high CTN/high density region near the outer part of the core may indicate barite mud invasion. This procedure is an excellent way to verify mud invasion and estimate its extent.
Quantitative CT scanning of cores requires modifications to the techniques employed for medical applications. The CT scanner must be tuned for reservoir rocks rather than water in order to obtain quantitatively correct measurements of CT response of the cores. Since repeat scanning of specific locations in the sample is often necessary, more accurate sample positioning is required than is needed in medical diagnostics.